1. Technical Field
The present invention relates to network delay measurements and, in particular, to correcting errors in end-to-end network delay measurements between two communicating data processing systems. Still more particularly, the present invention provides a method, apparatus, and program for correcting errors in network delay measurements due to clock skew and clock resets.
2. Description of Related Art
Packet delay traces are important measurements for analyzing end-to-end performance and for designing traffic control algorithms in computer networks. These measurement data can help in decision making in traffic routing, capacity planning, application tuning, alarm detection and network fault detection, etc. These delay traces can be obtained either by monitoring tools or by active probing. In either case, time stamps of packets are collected at the source and the destination. The difference between the two time stamps of the same packet is the measured delay for the end-to-end network delay experienced by that packet. If the two host clocks are perfectly synchronized, then the measured delay is the true delay. However, in real measurements, the two host clocks are usually not synchronized. In particular, the two clocks may run at different speeds. This difference in speed is called the “clock skew.” It is therefore possible for the receiver to receive a packet from the “future,” resulting in a negative delay according to the measurement. The measured delay in this case would be very different from the true delay.
The problem of estimating and removing the relative clock skews from delay measurements becomes more challenging and complicated if the clocks are reset through system calls such as “rdate.” Such resets are typically performed at a very coarse level, for example, a couple of times a day. Without prior knowledge of the reset times, one must “detect” them from the data, and obtain the “correct” delay measurements. Another type of resets is velocity adjustments through the use of Network Time Protocol (NTP). Such velocity adjustments are usually performed at a finer time granularity.
Therefore, it would be advantageous to provide an improved method, apparatus, and program for clock synchronization for network measurements with clock resets.